I need to upload a 100 dollar gift card so I can purchase more music.

Hello, depending on the device your using, please refer to the following steps below to redeem your gift card,

Looking at your post you're wanting to use windows follow these steps below.

Redeem a gift card on a Windows PC

1. Find the 16-digit code on the back of the card. For some gift cards, you might need to peel or gently scratch off the label to see the code.
2. Open iTunes.
3. From the menu bar at the top of the iTunes window, choose Account.
4. Choose Redeem.
5. Sign in and follow the instructions on the screen.

Redeem a gift card with your iPhone, iPad, or iPod touch

1. Find the 16-digit code on the back of the card. For some gift cards, you might need to peel or gently scratch off the label to see the code.
2. On your iPhone, iPad, or iPod touch, open the App Store app.
3. At the top of the screen, tap the sign-in button  or your photo.
4. Tap Redeem Gift Card or Code. If you don’t see Redeem Gift Card or Code, sign in with your Apple ID.
5. Tap Use Camera and follow the instructions on the screen. If you have trouble redeeming the card, tap Enter Code Manually, then follow the instructions on the screen.
6. Tap Done.